How do you make plastic take a walk? Purple.

Your Random Science News Story From the Month of June

Have you ever heard of the Stransbeest? It’s a kinetic sculpture that walks along a sandy beach thanks to the power of wind. Designed by Dutch artist Theo Jansen, he considers the PVC creations “new forms of life”. Here’s a TED Talk he did back in 2007:

This news story isn’t about his ‘Beach Beasts’, but I did immediately think of these creations after hearing that scientists had designed a new material that can crawl along – not thanks to the wind, but by light. You can read the original paper, published in Nature, if you have a subscription. If not, well, there are other ways…

What happened?

A team from Eindhoven University of Technology and Kent State University, led by Dick Broer, has designed a special new polymer that “undulates” when exposed to violet light.1 The strip in the above video – roughly the size of a small paper clip (22 by 4 mm) – is attached to a frame in order to turn that undulation into forward/backward movement. The team also designed and tested other shapes; those, however, require a non-fixed light source to properly move. One of them even has legs, which you can watch, here.

The framed strips crawl along at a speed reminiscent of the caterpillars they resemble – a mere half centimeter per second. But how do they move? The polymer isn’t uniform – the two sides (think two strips of gum stacked on top of one another, except only 0.02 mm thick) respond differently to the light at the same time:

“On illumination, the planar side shrinks strongly along the long axis of the film and expands weakly along the other two axes, causing the light-exposed area to curve downwards. In contrast, the homeotropic side shrinks strongly along the thickness axis of the film and expands weakly along the other two axes, causing the light-exposed area to curve upwards.” ~ Gelebart et al. 2017

This is not entirely dissimilar to how analog thermostats work – a bimetallic strip inside the device curls in response to temperature changes, because the two metals utilized expand at different rates to the same temperature.

In this case, the polymer is made up of long, light-sensitive molecules called azobenzene, but they’re arranged differently on each side of the strip. On the “planar” side, the molecules are parallel along the length of the strip; on the “homeotropic” side they’re perpendicular.

The strip is nearly transparent at visible wavelengths, but absorbs wavelengths of light on the border where we have to separate violet from ultraviolet.2 Photons have energy – violet the highest of the visible wavelengths – and by absorbing those photons the azobenzene molecules gain that energy and, in response, change shape.

The resulting deformations – one side overall contracting while the other expands – cease as soon as area is no longer exposed to the light, and the azobenzenes switch back to their original shape.

With the strip stuck inside a frame shorter than itself, it already starts off slightly curved; when the 405-nm LED light is shone from the front – and at a relatively small angle – that end starts bulging downward. This exposes the area of the strip behind it to the light, so then that part starts bulging downward, and this continues down the length of the strip producing a moving wave. It’s this moving wave that propels the strip away from the light.3

If the light is at too high an angle, the entire strip becomes exposed and the “self-shadowing effect” – which is required for the wave to travel along the strip – isn’t observed (i.e. you get a ‘bump’ but no movement).

 

So What?

This is not the first polymer to respond to light, but it is the first that only requires a steady source of light. Other materials also need heat, or humidity, or an application of light to different sections in order to move in a desired direction.4

lightwalker
Figure 4, Gelebart et al.

The researchers claim one possible use of these strips is to keep solar cells clean. When covered in dust, the efficiency of solar cells is naturally diminished, so any autonomous method to keep them clean will come as a welcome improvement to the system. The tested strips were capable of launching grains of sand off of them (see above image, column “a”) – or at least moving them to one, far side. Unlike the video above, the strips wouldn’t be set inside a personal walker, but attached to a surface at both ends. In the tests, they were literally taped down, but I assume for commercial uses it might be slightly less rudimentary.

Also, the strips were capable of transporting matter on top of them uphill, despite said matter being noticeably heavier (The paper doesn’t specify how much – they just say “much heavier and larger in size”). The strips inside the frames – that is, the ones demonstrated to be capable of continuous forward/backward motion – could be used to pull “masses of a few milligrams” over a few centimeters. That doesn’t sound all that impressive, but it could be useful for maneuvering small things into similarly small and poorly accessible areas.

It’s not ferrets pulling wires down the lengths of airplanes, but I guess it’s cool in its own way. Azobenzene won’t randomly take naps in the middle of a job.

 


1. Slightly ultraviolet light was also tested.

2. Thus, you can see a slight shadow beneath it.

3. If you flip the strip over, the process proceeds in the opposite direction, so the strip crawls toward the light source.

Also, potentially interesting side note: Upon exposure, an illuminated section of the polymer can reach as high as 50°C, but when the dent reaches the end and a new wave starts to propagate down the strip, it can get nearly as hot as the boiling point of water!

4. When you hear about movement caused by light, you might also think of solar sails, which are used to accelerate spacecraft by using the teeny tiny pushing force generated by photons when they hit the sail – a simple application of Newton’s 3rd law of motion. Different topic.

 

Cover image credit: Bart van Overbeeke

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